The overall goal of this project is to achieve a better understanding of the processes whereby interventions are translated into actions: 'executive' processes. The specific aim of this project is to study the executive processes involved in 1) switching from one task to another, and 2) preparing such a switch. These are important aspects of executive processes inasmuch as normal humans switch between tasks frequently and easily whereas specific patient populations are unable to: their actions become 'captured' by the stimuli in the environment. We proposed to address this issue by studying the electrophysiological (ERP) correlates of the executive processes involved in task-switching in normal subjects. On every trial, a cue will be presented that will inform the subject as to the task that is required on that trial. After one or two cue-to-target intervals (CTIs), a stimulus comprised of a letter and a number will be presented. When the cue is a red disk, subjects will judge whether the letter is a vowel or consonant; when the cue is a purple disk, they will judge whether the number is even or odd. One CTI will be long (1000 ms) allowing subjects ample time to prepare for the forthcoming trial; the other will be short (50 ms) and will not allow time to prepare. Thus, subjects will be required to randomly switch between a letter categorization task and will not allow time to prepare. Thus, subjects will be required to randomly switch between a letter-categorization task and a number-categorization task, and will be allowed to either prepare for this switch (or repeat) "ahead of time" or not. Their ERPs, as well as their behavioral performance (reaction times [RTs[ and error rates) will be recorded. This will allow us to compare subjects' ERPs and behavior across four conditions: 1) trials on which subjects switch from one task to the other ('switch' trials) and following a long CTI; 2) switch trials following a short CTI; 3) trials on which subjects repeat the same task they performed on the previous trial ('repeat' trials) following a long CTI; 4) repeat trials following a short CTI. Furthermore, this design will allow us to compare ERPs associated with the cue across the same four conditions. These latter comparisons will reveal the componentry of the processes associated with preparation to switch or repeat. The results of these comparisons will shed light on the neural mechanisms underlying the executive processes involved in task switching. High-density ERP recording will be conducted to permit precise localization in time, and grosser localization in space. Building on earlier work, we expect to find a network of frontal and parietal areas implicated in switching between tasks. While we have gone some way towards establishing the time- course of the activity in this network on switch trials relative to repeat trials, we have yet to understand when the different brain areas come into play when subjects are preparing to switch. This is a central, critical aspect of executive control, and one that the present proposal will elucidate.